This invention relates generally to well pumping units and more particularly to pumping units without walking beams. This invention employs an electric motor as its prime mover. A pair of crank mechanism symmetrically situated near the outside of the lower portion of a tower is used to generate a linear reciprocating movement in an approximately simple harmonic fashion. The distance of such movement is multiplied by multiplying means consisting of an assembly of fixed pulley blocks and a traveling pulley block. The degree of movement depends upon the number of pulleys in each pulley block. The movement is then transmitted to a counterweight-polished rod system connected to the multiplying means, making the polished rod reciprocate in a long distance, with approximately simple harmonic fashion. The effective length of the crank can be easily adjusted to obtain a required stroke. Also, by changing the number of pulleys operating in the multiplying means, the stroke distance can be changed to a greater extent. The stroke rate is changed by changing the motor speed or changing the transmitting ratio between the electric motor and the gear reducer.
A review of prior art will be helpful to the understanding and evaluation of this invention before a detailed description is given. At present, pumping units are widely used for lifting oil from those wells which are not gushers. As the liquid level in the well becomes lower as the time as production goes on, or as the need to lift oil therefrom deeper wells arises, a greater depth of plunger is required. This results in some problems, the first of which is a lower pump efficiency. Since the metal sucker rod is elastic, it will be stretched and the load applied on it, resulting in a stroke loss. The greater the depth of plunger, the greater the stroke loss will be, meaning a lower pump efficiency. If the stroke of a pumping unit is longer, however, the relative stroke loss is less, so it is favourable to use a long stroke pumping unit in such cases. Long stroke pumping units are especially good for wells producing oil of high viscosity or of high water-oil ratio. For a certain production rate, a longer stroke distance means a lower stroke rate, consequently a lower speed and a lower acceleration, thus providing various advantages and benefits. For example, a longer fatigue life of the sucker rod is provided because it experiences reduced cycles of stresses of variable amplitude. Another benefit is the reduced wear of the plunger and tubing because of the lower speed and acceleration of operation and the greater area over which the wear is distributed. In brief, A longer stroke makes a higher pump efficiency and a longer service life of various parts.
Long stroke pumping units can be classified into two types by their designs, i.e., walking beam pumping units and pumping units without walking beams. A walking beam long stroke pumping units is shown in U.S. Pat. No. 4,306,463, comprising a walking beam pivotally mounted for free swinging motion on a Sampson post. The walking beam includes a fixed counterweight mounted on one end and a horsehead mounted on the opposite end. A polished rod is connected to the horsehead through a wire rope. The walking beam is driven by an electric motor or by hydraulic means in an up and down pumping motion, making the polished rod move up and down. The shortcomings of walking beam pumping units are the cumbersome structure, heavier weight, greater base requirement and existence of centrifugal force generated by the rotating counterweights.
A typical long stroke pumping unit without walking beam includes an upright tower with a rotatable drum mounted on its top. A wire rope or a belt winds around the drum with the polished rod connected to it on one end and the counterweight connected to the opposite end. A winch driven by an electric motor or a hydraulic motor moves, directly or indirectly, the polished rod. Motion of the polished rod may be reversed by reversing the electric or hydraulic motor. The deceleration, or dampening, of motion near the termination of the stroke is realized electrically or hydraulically.
U.S. Pat. No. 3,285,081 shows one kind of long stroke pumping unit without walking beams. It comprises a tower with a drum mounted on its top, a counterweight and a rod string suspended from said drum but moving in opposite directions. A driving system comprising a reversible electric motor and a gear reducer and a wire rope connecting the output shaft of the gear reducer and the drum makes the drum rotate in alternative directions. The stroke distance of the pumping unit depends on revolutions of the electric motor during the stroke. The motion reversal of the polished rod and the deceleration and dampening upon reversal of the motion may be accomplished by reversing, stopping, and starting the electric motor electro-mechanically.
For other prior arts related to this invention, also see U.S. Pat. Nos. 3,538,777. 3,695,117, 3,771,609, 4,052,907, 4,062,640, 4,388,837, 4,391,155.